1. Field of the Invention
This invention relates to musical tone synthesis and in particular is concerned with an improvement for producing tones whose spectra are touch responsive to the actuation of keyboard switches.
2. Description of the Prior Art
A wide variety of musical tone generation systems have been designed which attempt to realistically replicate the sounds produced by conventional acoustic musical instruments. In general these systems have produced only poor imitative sounds because they lack the capability to produce the complex time variations of the musical waveform that characterize a tone from a particular acoustic musical instrument. The most obvious method to imitate a musical instrument is to record the sound and to replay these recordings in response to an actuated keyswitch.
While at first glance the straightforward technique of recording and keyed playback seems to be attractive, a practical realization of such a musical instrument can be burdened by a large amount of memory for storing the recorded data. The maximum amount of storage is associated with a system that uses a separate and distinct recording for each note played in the range of the musical instrument's keyboard. Some economy in storage has been made by using a single recording for several contiguous notes making use of the assumption that the waveshape for the imitated musical instrument does not change greatly between successive notes.
Electronic musical tone generators that operate by playing back recorded musical waveshapes stored in a binary digital data format have been given the generic name of PCM (Pulse Code Modulation). This is an unfortunate label because PCM can mean almost anything. In particular PCM in no way simply identifies the tone generator as one in which a recorded tone is simply stored in a binary digital data format. A musical instrument of the PCM type is described in U.S. Pat. No. 4,383,462 entitled "Electronic Musical Instrument." In the system described in the patent the complete waveshape of a musical tone is stored for the attack and decay portions of the musical tone. A second memory is used to store the remainder of the tone which comprises the release phase of the musical tone. The sustain phase of the tone is obtained by using a third memory which stores only points for a single period of a waveshape. After the end of the decay phase, the data stored in the third memory is read out repetitively and the output data is multiplied by an envelope function generator to create the amplitude variation for the sustain and release phases of the generated musical tone.
It is an inherent characteristic of most orchestral type musical instruments that the spectral composition of the tone is a function of the loudness with which the instrument is played. If such loudness spectral variations are to be imitated in a simple data storage type PCM tone generation system, the number of waveshape memories would have to be increased. The large amount of memory would make it difficult to implement such a system at a low enough cost to produce a commercially viable instrument.